Elsevier

Computers in Biology and Medicine

Volume 58, 1 March 2015, Pages 91-100
Computers in Biology and Medicine

Quantitative assessment of responses of the eyeball based on data from the Corvis tonometer

https://doi.org/10.1016/j.compbiomed.2015.01.006Get rights and content

Highlights

  • The proposed algorithm enables to determine responses of the eyeball to an air puff.

  • Responses of the eyeball can be linked to some features of corneal deformation.

  • Using profiled algorithm enables to measure additional parameters from the Corvis.

Abstract

Background

The “air-puff” tonometers, include the Corvis, are a type of device for measuring intraocular pressure and biomechanics parameters. The paper attempts to analyse this response and its relationship with other parameters measured in the Corvis tonometer.

Methods

A number of 13,400 2D images were acquired from the Corvis device and analysed (32 healthy and 16 ill people). A new method has been proposed for the analysis of responses of the eyeball based on morphological transformations and contextual operations.

Results

The proposed algorithm enables to determine responses of the eyeball to an air puff coming from the Corvis tonometer. Additionally, responses of the eyeball have been linked to some selected features of corneal deformation. The results include, among others: (1) distinguishability between the left and right eye with an error of 7%; (2) the correlation between the area under the curve in corneal deformation and the response of the eyeball −0.26; (3) the correlation between the highest concavity time and the maximum deformation amplitude of 0.4. All these features are obtained fully automatically and repetitively at a time of 3.8 s per patient (Core i7 10 GB RAM).

Discussion

It is possible to measure additional parameters of the eye deformation which are not available in the original software of the Corvis tonometer. The use of the proposed methods of image analysis and processing provides results directly from the eye response measurement when measuring intraocular pressure.

Introduction

The rapid advances in technology and, in particular, methods of image analysis and processing enable to perform a fully automatic measurement of intraocular pressure (although the measurement is not considered the gold standard). One such device is the Corvis tonometer [1], [2]. Apart from the measurement of intraocular pressure, it also allows for the measurement of other parameters of the eye [3]. The measurement method involves corneal flattening forced by an air puff—Fig. 1. Next, the corneal contour image analysis enables to measure the corneal thickness, deformation amplitude, applanation length, rate of corneal deflection and intraocular pressure [4], [5]. They are shown in Fig. 2. In addition, the information about applanation points, pachymetry, and others is provided [6], [7]. For example, determination of temporary states of corneal flattening during a convex/concave transition and vice versa enables to designate two moments in time [3], [5]. For these moments, the pressure value is read and then the mean value is calculated. This value after linear calibration is the IOP value. There are also other parameters of corneal deflection calculated in the Corvis tonometer (Fig. 2).

Modern methods of image analysis and processing allow for the measurement of a much larger number of parameters than those that are available in the Corvis tonometer and proprietary software. Their determination requires the use of the Roberts, Sobel and Prewitt edge detection methods [8]. Morphological operations such as opening and closing, also treated as conditional, are useful here. The created software is characterized by much greater functionality, full access to data and almost limitless possibility of further processing, in particular statistical processing. This is particularly important because in the current version of the Corvis software it is not possible to access data, e.g. the results of the designated corneal contour. Additionally, the contour is not designated in the full range of the cornea visible in the image.

Groups of studies related to the analysis of biomechanical parameters of the cornea for the ORA tonometer [9], [10] and the effect of patients’ age [11], [12], [13], [14], glaucoma [15] or wound healing [16] on the results obtained are known from the literature. Predictive numerical simulation of corneal biomechanical properties was considered in an equally interesting way [17]. The methods for measuring the ultrastructure of the corneal stroma are presented in paper [18]. There are also very interesting studies profiled to the analysis of the cornea in Brazilian patients [19] or children with the use of the ORA tonometer [20], [21]. There also exist publications which show the correlation between the hysteresis obtained using the tonometer and pachymetry [22]. By contrast, Congdon et al. [23] present hysteresis in correlation with glaucoma damage. The paper [24] deals with these issues (hysteresis) but in the Reichert ocular response analyser. Gatinel et al. [25] draw attention to the corneal hysteresis, resistance factor and topography. There is also a known group of studies related to the analysis of biomechanical parameters of the cornea in keratoconic eyes, [26], [27], or hypertension and glaucoma [28]. Also other examinations of the cornea are described which certainly contribute to the understanding of the biomechanical properties, for example, the corneal strip extensometry comparison presented in the paper [29]. In a review of the literature there are no studies related to the analysis of corneal images from the Corvis tonometer and, in particular, to the analysis of responses of the eyeball based on these images. Only in the paper [30] the authors show the division of the reaction to an air puff into the eyeball response and corneal response. The eyeball response itself is not analysed in the context of other biomechanical parameters, especially those available in the original software of the Corvis tonometer. The same applies to the paper [31] which shows the algorithm designed to acquire additional parameters from the Corvis tonometer, but without correlation analysis with the existing, well-known and aforementioned parameters. For this reason, this publication shows: (1) new biomechanical parameters of the eyeball response measured during corneal deformation; (2) correlation of the determined biomechanical parameters with the parameters measured by the original software of the Corvis tonometer; (3) analysis of results and evaluation of their practical usefulness.

Section snippets

Material

For the purposes of analysis, images with an M×N×I resolution of 200×576×140 pixel were acquired from the Corvis device in the source recording format *.cst. The patients ranged in age from 17 to 63 years. They were healthy (32 people including 16 women) or ill (16 people including 9 women). The ill ones were patients with AMD (age-related macular degeneration) or other diseases that cause abnormal pressure in the eye. For each patient, there were 140 images in a sequence, which for 96 eyes gave

Method

The proposed new method for the analysis of data from the Corvis tonometer was divided into two stages.

The first stage, namely image pre-processing, involved the analysis of data from the Corvis tonometer, which enabled the reconstruction of corneal shape changes and the separation of the eyeball response.

In the second stage, namely data processing, a detailed analysis of the response of the eyeball and its correlation with other parameters of the patient was performed.

Results

The results of a preliminary correlation between the features from w(1) to w(4) and from v(1) to v(14) obtained for 96 eyes are shown in the form of an image in Fig. 8. These results indicate that the highest correlation value (absolute values) is obtained for the correlation of v(1) with w(1), v(5) with v(4), v(11) with v(5), v(11) with v(14), v(11) with v(6) etc.—Table 2. The resulting correlation values will be subject to further description. The selection of only the highest correlation

Conclusions

The paper presents a fully automatic method for measuring responses of the eyeball calculated on the basis of data from the Corvis tonometer. In addition, a set of 4 different features, relevant from a practical point of view of the eyeball response assessment, was proposed. Moreover, the obtained results of responses of the eyeball were compared with the results of corneal reactions. The presented methodology of image acquisition and acquisition of individual parameters is not exhaustive of

Summary

The “air-puff” tonometers, include the Corvis, are a type of device for measuring intraocular pressure and biomechanics parameters. Additionally, one of the parameters obtainable from the Corvis tonometer using the software proposed by the authors is the response of the eyeball to an air puff. The paper attempts to analyse this response and its relationship with other parameters measured in the Corvis tonometer.

13,400 2D images were acquired from the Corvis device and analysed (32 healthy and

Abbreviations

ROI, region of interest; IOP, intraocular pressure.

Competing interests statement

The authors declare that they have no competing interests.

Authors’ contributions

RK suggested the algorithm for image analysis and processing, implemented it and analysed the images. EW, SW, AN, ST, AB, ZW performed the acquisition the images from Corvis and consulted the obtained results. All authors have read and approved the final manuscript.

Acknowledgements

No outside funding was received for this study.

References (44)

  • S. Shah et al.

    The use of the Reichert ocular response analyser to establish the relationship between ocular hysteresis, corneal resistance factor and central corneal thickness in normal eyes

    Contact Lens Anterior Eye

    (2006)
  • D. Ortiz et al.

    Corneal biomechanical properties in normal, post-laser in situ keratomileusis, and keratoconic eyes

    J. Cataract Refract. Surg.

    (2007)
  • P. Bañeros-Rojas et al.

    Comparison between Goldmann, Icare Pro and Corvis ST tonometry

    Arch. Soc. Esp. Oftalmol.

    (2014)
  • J. Hong et al.

    A new tonometer—the Corvis ST tonometer: clinical comparison with noncontact and Goldmann applanation tonometers

    Invest. Ophthalmol. Vis. Sci.

    (2013)
  • B.F. Valbon et al.

    Ocular biomechanical metrics by CorVis ST in healthy Brazilian patients

    J. Refract. Surg.

    (2014)
  • A. Smedowski et al.

    Comparison of three intraocular pressure measurement methods including biomechanical properties of the cornea

    Invest. Ophthalmol. Vis. Sci.

    (2014)
  • M. Lanza et al.

    Evaluation of corneal deformation analyzed with scheimpflug based device in healthy eyes and diseased ones

    BioMed Res. Int.

    (2014)
  • S. Bak-Nielsen et al.

    Dynamic Scheimpflug-based assessment of keratoconus and the effects of corneal cross-linking

    J. Refract. Surg.

    (2014)
  • R.C. Gonzalez et al.

    Digital Image Processing Using Matlab

    (2008)
  • A.S. Kobayashi et al.

    Viscoelastic properties of human cornea

    Exp. Mech.

    (1973)
  • E.S. Sherrard et al.

    Age-related changes of the corneal endothelium and stroma as seen in vivo by specular microscopy

    Eye (London)

    (1987)
  • A. Kotecha et al.

    Corneal thickness and age-related biomechanical properties of the cornea measured with the ocular response analyzer

    Invest. Ophthalmol. Vis. Sci.

    (2006)
  • Cited by (11)

    • Corneal deformation amplitude analysis for keratoconus detection through compensation for intraocular pressure and integration with horizontal thickness profile

      2019, Computers in Biology and Medicine
      Citation Excerpt :

      This data is limited to the central horizontal cross-section of the cornea, which is different from the 3D tomography and topography measured by videokeratography devices. Earlier studies have evaluated corneal deformation recorded in Corvis ST images and separated the movement of the eye [18] from corneal response [19], here called the corneal deflection amplitude (DA). In other earlier studies, corneal deformation led to estimation of a biomechanically-corrected IOP (bIOP) that was shown to be less affected by corneal biomechanical parameters than the Corvis ST reading [20,21].

    • Quantitative assessment of corneal vibrations during intraocular pressure measurement with the air-puff method in patients with keratoconus

      2015, Computers in Biology and Medicine
      Citation Excerpt :

      The left eye reacts differently in comparison with the right one [16,17]. These reactions are so characteristic that on their basis it is possible to say with almost 100% certainty which eye (left or right) is being examined [18,19]. Extremely low-frequency corneal deformations are primarily three characteristic moments: the time of the first applanation (temporary flattening of the cornea), the maximum corneal deformation, the time of the second applanation [20].

    • Air-puff-induced dynamics of ocular components measured with optical biometry

      2019, Investigative Ophthalmology and Visual Science
    View all citing articles on Scopus
    View full text